Electro Permanent Magnet Gripper (OpenGrab EPM v3)

This article shows how to use the Electro Permanent Magnet Gripper (OpenGrab EPM v3) with ArduPilot.

Overview

The OpenGrab EPM v3 (from NicaDrone.com) is a small (less than 4cm x 4cm x 2cm) Electro Permanent Magnet gripper capable of 200N of force meaning it can hold about 15kg of cargo securely. The gripping and releasing of the cargo is accomplished by delivering a short pulse to an electromagnet which reverses it’s field. Objects that it grips onto should be metal or have the included thin steel target square attached to them.

The device accepts a standard PWM servo input or UAVCAN and uses a 5V power supply. A significant amount of power is only required for about 1 second when it first grips or releases the cargo.

Connecting to a Pixhawk via UAVCAN

../_images/OpenGrabEPMV3_2.jpg

Notes:

  • When Vin of the EPM drops below 5V the EPM will go into low power mode. Slowing down switching to prevent crashing the power rail.
  • When Vin drops below 4.5V the green status LED will blink fast indicating an error condition.
  • The Pixhawk UAVCAN connector power rail voltage is < 5V.
  • External powering of the EPM is recommended (see image above).
  • EPM UAVCAN cable set is required. Clickmate -> DF13.
  • UAVCAN requires termination, terminator included in UAVCAN cable set.

Firmware setup

Note

Auto mission using PWM works with current Copter release, this step may be skipped. UAVCAN support requires Copter-3.4A or later.

At the moment Copter 3.4A is not released. It can be built from the master. or:

  1. Download APM firmware here:.
  2. Upload custom firmware via Mission Planner or QGC.

Parameter setup

  1. Set BRD_CAN_ENABLE = 2 (Enable support for dynamic node ID allocation and firmware update). Save and restart the board. After about 5 seconds the RED CAN LED on the EPM will start blinking. This indicates that CAN frames are being received.

    ../_images/OpenGrabEPMV3_3.jpg

  2. Set CH7_OPT: Channel 7 option = 19 (EPM) (Channel 7 - 12 maybe used)

    ../_images/OpenGrabEPMV3_4.jpg

All done! The EPM can be controlled via channel 7 or AUTO mission or MAVLink command.

Connecting to a Pixhawk via PWM header

Connect the EPM to the Pixhawk the same as a servo.

../_images/OpenGrabEPMV3_5.jpg

Parameter setup

Set CH7_OPT: Channel 7 option = 19 (EPM) (Channel 7 - 12 maybe used).

../_images/OpenGrabEPMV3_6.jpg

All done. The EPM can be controlled via channel 7 or AUTO Mission or MAVLink command.

Auto mission

Add a Do Gripper command in your mission.

../_images/OpenGrabEPMV3_7.jpg

Magnetic interference and board voltage check

The EPM can cause magnetic interference especially while gripping or releasing the cargo or if left in the “on” state with no cargo attached. Magnetic interference is minimal at a distance of 10 cm

Testing the EPM impact on the compass is highly recommend when mounted close then 10 cm distance from flight computer

The EPM’s impact on the compass can be tested in real-time by connecting with the Mission Planner. Go to the Flight Data screen and click on the “Tuning” checkbox at the bottom, middle. Double click on the graph that appears on the top right and when the “Graph This” window appears select “magfield”.

Turning the EPM on and off and ensure the magfield length does not change by more than 10%.

../_images/mag_field.jpg